Xtreme Link Experiences

[tadawson]

	Quote: Originally Posted by dirtybird
	When I was designing test instrumentation for RF measurements of very large antenna I sure wished what you say were true. I suggest you take a look at "Principles of Radar" by J. Scolnik ( I hope I spelled his name right. Its been a long time). Now I know that's an old book but the fundamentals are the same. A parabolic dish will concentrate all of the energy toward one path. You will then get close to the transmitter radiated power but not more than that in the near field. ERP is the equivalent power you will see with antenna equipped radiator over that of a simple Dipole antenna. Even with a parabolic dish you must be in the far field to get an accurate measurement. The far field is defined as the distance equal to at least 10 times the wavelength. 100 time the wavelength is better by quite a bit. We don't have a parabolic dish on our systems. They woouldn't work well as they are too directive.

Uh, duh! I was trying to simplify things, since it appeared that you weren't getting it. Sorry to let you know, but 5 years engineering 900MHz to 2.4 GHz with Motorola pretty much verified for me that antenna gain is a very real, usable, fact of RF engineering. It is true, as you say, that we can't use antennas with much gain, since the nulls in the pattern become prohibitive (and I think I said that early on) but then again, my comment was not about the practicality of gain antennas, but rather the reality of same.

Oh, and on the older 900MHz cellular systems, transmitter power was a whopping 5 watts max, and TX antenna gain up to 26 db . . . . worked like a champ, and got us to the FCC max ERP of 100 watts, IIRC . . . . heck, with some of the systems, we had to pad the transmitters, since the gain put us well over the limit!

- Tim

[dirtybird]

[quote=tada
Oh, and on the older 900MHz cellular systems, transmitter power was a whopping 5 watts max, and TX antenna gain up to 26 db . . . . worked like a champ, and got us to the FCC max ERP of 100 watts, IIRC . . . . heck, with some of the systems, we had to pad the transmitters, since the gain put us well over the limit!

- Tim[/quote]
You never had more than 5W of radiated power anywhere. What you had was the equivalent power of a 100 wattt transmitter radiating with a 1/2 wave dipole antenna. Again you have to be in the far field to measure that.
Otherwise notify Al Gore that you have found the secret to Global Warming. You can generate power with a simple parabolic dish that will generate no carbon dioxide residue.

[tadawson]

	Quote: Originally Posted by dirtybird
	You never had more than 5W of radiated power anywhere. What you had was the equivalent power of a 100 wattt transmitter radiating with a 1/2 wave dipole antenna. Again you have to be in the far field to measure that. Otherwise notify Al Gore that you have found the secret to Global Warming. You can generate power with a simple parabolic dish that will generate no carbon dioxide residue.

Apparently you are stuck on semantics, or just plain don't get the concept of ERP whatsoever. You are thinking of summing all power coming off the antenna, in all directions, which yes, can never exceed the true TX output power (minus TX line losses). This, however, is naive, and totally missed the point of gain antennas . . . .

The FCC doesn't care about the actual power coming off the TX in this case - they only care about the field strength radiated. So, if a 5w tx with a large gain antenna gives field strength X, and an 100w tx with a dipole gives the same, the the difference in this case is purely irrelevant. The idea behind directional antennas is not to waste power radiating signal where it is not needed - IE, on a cellular system, very few phones are straight up, or underground, so using directional antennas gives the same radiated field as a dipole, without the power waste. So, net result of 100w tx/dipole and 5w tx/gain antenna is the *SAME*. Nobody gives a rat's butt that the actual power off the TX is different, since in the radiated field, it's dead-on balls the *SAME*. Get it? If not, I am not going to waste my time in any more remidial RF engineering training - this is really, really, simple stuff . . . .

As applied to R/C, your aircraft should never be directly overhead or behind you, so there is *some* potential for antenna gain, although we all have those oh-$*** moments when things get where they should not be . . . And frankly, very few of us ever get to the limits of our systems. So a 3db or 6db gain antenna could be used to give a bit more range, but then again, it relies on the user to *NOT* point the antenna at the model, since the null will be deeper!

- Tim

[JimDrew]

	Quote: Originally Posted by tadawson
	Apparently you are stuck on semantics, or just plain don't get the concept of ERP whatsoever. You are thinking of summing all power coming off the antenna, in all directions, which yes, can never exceed the true TX output power (minus TX line losses). This, however, is naive, and totally missed the point of gain antennas . . . . The FCC doesn't care about the actual power coming off the TX in this case - they only care about the field strength radiated. So, if a 5w tx with a large gain antenna gives field strength X, and an 100w tx with a dipole gives the same, the the difference in this case is purely irrelevant. The idea behind directional antennas is not to waste power radiating signal where it is not needed - IE, on a cellular system, very few phones are straight up, or underground, so using directional antennas gives the same radiated field as a dipole, without the power waste. So, net result of 100w tx/dipole and 5w tx/gain antenna is the *SAME*. Nobody gives a rat's butt that the actual power off the TX is different, since in the radiated field, it's dead-on balls the *SAME*. Get it? If not, I am not going to waste my time in any more remidial RF engineering training - this is really, really, simple stuff . . . . - Tim

This is inaccurate. FCC measures the radiated power at the RF module itself. I know for fact - we are doing FCC testing right now for our new hardware. Antenna gain is calculated in addition to your module and only antennas meeting the defined energy levels are allowed. This prevents someone from deliberately using a poorly designed antenna for FCC approval and then using a good antenna to get extra power without needing additional approval (and likely being over on spectral density).

[tadawson]

	Quote: Originally Posted by JimDrew
	This is inaccurate. FCC measures the radiated power at the RF module itself. I know for fact - we are doing FCC testing right now for our new hardware. Antenna gain is calculated in addition to your module and only antennas meeting the defined energy levels are allowed. This prevents someone from deliberately using a poorly designed antenna for FCC approval and then using a good antenna to get extra power without needing additional approval (and likely being over on spectral density).

Jim -

Please read the ENTIRE thread before you post incorrect comments like this one. I was speaking about the 900MHz CELLULAR PHONE SERVICE, and *NOT* R/C! In that service, the FCC does not measure TX output at all - the line losses and such can be so high on tall towers that they *ONLY* regulate ERP so that a system can run at full power, and run whatever transmitters and antennas they choose to reach that goal . . . .

I did the license filings to the FCC for these systems for Motorola back in the early 80's, so trust me, I know what the rules were . . . .

- Tim

[JimDrew]

	Quote: Originally Posted by tadawson
	Jim - Please read the ENTIRE thread before you post incorrect comments like this one. I was speaking about the 900MHz CELLULAR PHONE SERVICE, and *NOT* R/C! In that service, the FCC does not measure TX output at all - the line losses and such can be so high on tall towers that they *ONLY* regulate ERP so that a system can run at full power, and run whatever transmitters and antennas they choose to reach that goal . . . . I did the license filings to the FCC for these systems for Motorola back in the early 80's, so trust me, I know what the rules were . . . . - Tim

Well Tim, perhaps you know what the rules were but things have changed from the 80's. I never said we were working with 2.4Ghz R/C products.

[tadawson]

	Quote: Originally Posted by JimDrew
	Well Tim, perhaps you know what the rules were but things have changed from the 80's. I never said we were working with 2.4Ghz R/C products.

Different services have always had different regulations on things like this. Considering that 900MHz cellular is all but dead in the US, I doubt the rules have changed, but rather just aren't in play any more.

Bottom line is, there are several ways of regulating power, depending on service:

TX input power
TX output power - constant carrier or PEP
ERP
Antenna input power

And if I was bored enough, I could find sections of current FCC regulations that use all of them . . . . There is just not *one* way that things are regulated . . . .

(Well, I guess I am bored then . . . . this isn't for cellular, but is a clear, current, FCC reference to regulating power by ERP: http://edocket.access.gpo.gov/cfr_20...7cfr22.535.pdf.

And for cellular: http://edocket.access.gpo.gov/cfr_20...7cfr22.913.pdf. Note that ERP is the *ONLY* thing regulated in regard to transmit power . . . .

And here is the Amateur Radio Service - which regulated TX power only: http://edocket.access.gpo.gov/cfr_20...7cfr97.313.pdf Note that a HAM can run whatever antenna he chooses with it's correspondingly higher ERP and be totally legal - ERP is not limited in this service.

And if it's HAM radio R/C, then there *is* a TX power limit specific to that - 1watt TX out: http://edocket.access.gpo.gov/cfr_20...7cfr97.313.pdf Note that there does not appear to be any ERP restriction here either, so a HAM could run a gain antenna in R/C use, and have an ERP well over 1 watt . . .

72MHz R/C is limited to .75 watts here: http://edocket.access.gpo.gov/cfr_20...7cfr95.210.pdf)

- Tim